Camera not visible after changing units to mm in Blender
Select the camera object in the object list in the top right corner. Then press the forward slash key "/" to isolate the camera. This will hide everything but the camera and zoom in/center the viewport onto the camera.

From there, I'm sure you can figure out how to rescale and reposition the camera to fit the scene.

Changing the units and unit scale should not affect any of your models...I'm curious as to why that happened.